Based on the 2020 NEC
by Mark Lamendola
NEC Article 90 draws boundaries around the National Electrical Code—boundaries many people fail to understand. For example, Article 90 has long made it clear the NEC is not intended as design specification or instruction manual. The National Electrical Code has one purpose only. Logically, Article 90 belongs in Chapter 1 (along with Article 80)
90.1 has four subdivisions:
90.2 describes the scope of the Code—what it covers and what it does not cover.
90.3 explains how the Code is arranged. Please note the influence of the international and engineering communities. For example, the Code uses the "dot" system of enumeration and the "Appendices" are called "Annexes."
90.4 gives the Authority Having Jurisdiction (AHJ) some flexibility in enforcement.
90.5 distinguishes between mandatory rules, permissive rules, and explanatory material. These often get confused. An example is a Fine Protection Note (FPN) that discusses voltage drop. The Code does not require addressing voltage drop—it merely explains that it is an additional consideration and gives a "rule of thumb." Unfortunately, many people have over-engineered to get "the Code-required drop" or have under-engineered because they were "within the Code requirements." The Code does not give voltage drop requirements.
90.6 discusses formal interpretations.
90.7 adds a dose of common sense regarding equipment inspections. For example, a product that is Listed (e.g., by U.L.) can be assumed to be adequate for the stated purpose and need not be inspected again (except for alterations or damage).
90.8 alerts the user to allow for expansion and to know that the Code does specify various restrictions on the number of wires and circuits in a given enclosure.
90.9 discusses units of measurement.